This looks to be the study.
It's odd that the Guardian neither links to it, nor even names the researchers or their institution.
It's odd that the Guardian neither links to it, nor even names the researchers or their institution.
I wouldn't be surprised further study will show the last bit is due to earplugs at higher noise levels, resulting in a relative reduction in sleep disruption. Unless they also took earplug use into account already, then I'm genuinely curious what could be the reason